No problem, let's work through this together. We're starting with 9012 and 3866 and adding them all up.

Step 1: We'll start by adding the digits 2 & 6 in column 1 and get 8.

Step 2: We'll start by adding the digits 1 & 6 in column 2 and get 7.

Step 3: We'll start by adding the digits 0 & 8 in column 3 and get 8.

Step 4: We'll start by adding the digits 9 & 3 in column 4 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

So, 9012 + 3866 = 12878.

